This letter presents preliminary results of a study concerning the influence of forest thinning on the backscattering coefficient (sigma) at L-band by using data from the JPL-NASA campaign MAESTRO-1 and MacEurope on Les Landes forest, France. The backscattering coefficient of thinned and unthinned stands were compared and interpreted with the help of model simulations. It is shown that thinning significantly influences the backscattering coefficient of adult stands, particularly at HV polarisation where sigma was reduced by up to 4 dB. Moreover, the density variation of the stand did not explain all this decrease in sigma. Secondary structure modifications induced by the thinning are likely to have considerable influence on the backscattering coefficient.